1. Technical Field
The present invention relates to a ferrite composition for high frequency beads and a chip bead comprising the same and, more particularly, to a ferrite composition for a high frequency bead, which may be sintered without using any additive for low temperature sintering and be applicable at bandwidths of several hundreds MHz, and a chip bead comprising the same.
2. Description of the Related Art
In general, a magnetic ceramic component such as a multilayer type chip bead is mostly formed using magnetic materials such as nickel-zinc ferrite, nickel-zinc-copper ferrite, or the like. In order to increase sintering properties of the nickel-zinc ferrite, Cu is mostly added thereto, to prepare a three-component system composed of nickel-zinc-copper ferrite. Other than major ingredients of the three-component system, a sintering auxiliary agent such as Bi2O3 or a low melting point glass may be added. Alternatively, adding SnO2, Mn3O4, etc., may control characteristics of the ferrite.
The NiZnCu ferrite described above has a cubic spinel structure and is prepared by altering contents of NiO, ZnO, CuO and/or Fe2O3. A material having low permeability may be more utilized in the range of relatively high frequency bandwidths.
The following FIG. 1 shows a graph of relationship between permeability and frequency and, from this, it can be seen that the high permeability material is useable in the low frequency bandwidth, as compared to materials having low permeability.
Snoek's law may be expressed by the following Equation 1 and the NiZnCu ferrite entails a problem of non-use in a relatively higher frequency bandwidth due to restricted frequency properties caused by limits according to Snoek's law.
                    fr        =                                                          γ                                      ⁢                          M              S                                            3            ⁢                          π              ⁡                              (                                                      μ                    i                                    -                  1                                )                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Wherein fr denotes a self-driven resonance frequency, r is a magnetic constant, ui is an initial permeability, and Ms is a saturated magnetization value.
Accordingly, although the demand of chip parts capable of being used at relatively high frequencies is increased with the development of electronic instruments, NiZnCu ferrite generally used in the related art cannot be utilized due to limits according to Snoek's law. Therefore, there is a need for a solution to overcome such limitations as described above.
Furthermore, with regard to the fabrication of a multilayer type chip device using a ferrite composition in a bandwidth of several hundreds MHz, the multilayer type chip bead described above uses an internal electrode mostly comprising of silver (Ag). Since Ag has a melting point of 961° C., a ferrite composition calcined at a temperature of 961° C. or less may be required in order to form a chip. A material capable of being calcined at an appropriate calcination temperature of 920° C. or less, may be preferable.
Meanwhile, among materials overcoming limitations according to Snoek's law, a hexagonal type barium ferrite is known. Barium ferrite may include M, W, Y, Z-type ferrites depending upon contents of constitutional components. In particular, M-type has a chemical formula of BaFe12O19 and, likewise, W-type, Y-type and Z-type may have chemical formulae of BaMe2Fe16O27, Ba2Me2Fe12O22 and Ba3Me2Fe24O41 (Me═Co, NI, Zn, Cu and Mn), respectively.
With regard to the foregoing chemical formulae, it is known that the ferrite encounters more significant difficulties in preparation, as a chemical content of oxygen is increased. In case of M-type, the ferrite possesses ferromagnetic properties with high magnetic coercive force (‘coercivity’)(that is, hard ferrite), thus generally being used as a permanent magnet. However, it is known that the above ferrite is not being applied in chip beads due to ferromagnetic properties.
Among the foregoing, W, Y and Z-types show soft ferrite properties and, therefore, may be applicable at a high frequency bandwidth. However, since all of the M, W, Y and Z-types are substantially synthesized at 1200° C. or more, these are not suitable to use as a multilayer type chip bead material.
Next, FIG. 2 shows inductance values versus frequencies. An applicable region, in which the ferrite can be used as chip beads, is present at a frequency bandwidth in a flat area ahead of the self-resonance frequency (SRF). In order to allow the ferrite to be used (as chip beads) at a higher frequency, it is preferable to move SRF toward the high frequency side.